DISSECTION OF THE DOG 229 



The ventral sacro-iliac ligament (lig. sacro-iliacum ventrale) consists of short 

 fibres joining the sacrum and ihum, and Hes, as its name indicates, ventral to 

 the joint. Short and long sacro-iliac ligaments (lig. sacro-ihacum dorsale breve 

 et lig. sacro-iliacum dorsale longum) connect the ilium with the spinous 

 processes and the lateral border of the sacrum respectively. 



At some distance from the sacro-iliac joint, but conveniently considered 

 in this place, is the sacro-tuberous ligament (hg. sacro-tuberosum), which 

 stretches in the form of a stout cord from the edge of the sacrum to the 

 sciatic tuber of the ischium. 



Symphysis pelvis. — The two pubes and the two ischia join in the median 

 plane at the symphysis of the pelvis. Occasionally in old animals the union 

 becomes bony. A few transverse fibres, crossing the ventral aspect of the 

 symphysis, constitute the arcuate ligament (lig. arcuatum). 



Organon audiius. — It now remains for the dissector to complete the 

 dissection of the organ of hearing by an examination of the internal ear and 

 the cavity of the tympanum. To do this with any degree of satisfaction, it 

 is necessary to have several specimens, so that sections across some of them 

 may be made mth a fine saw, while one at least is examined after piece-meal 

 removal of parts of the bone. 



Prior to commencing the dissection of the internal ear and the cavity of 

 the tympanum, it would be well that the student have some general idea of 

 the auditory apparatus as a whole. The organ of hearing is naturally divided 

 into three parts : an external ear or auricle (auricula) , a middle ear or cavity 

 of the tympanum, and an internal ear. An examination of the cartilages of 

 the external ear and the muscles concerned in the production of their movements 

 was conducted at an earher stage of the dissection. The canal circumscribed 

 by the external ear is known as the external acoustic meatus (meatus acusticus 

 externus) and is partially bounded by the temporal bone. The meatus ter- 

 minates at the membrane of the tympanum (membrana tympani), by which 

 its cavity is separated from that of the tympanum. 



The cavity of the tympanum, or middle ear, is a space within the temporal 

 bone connected mth the interior of the pharynx by the auditory tube of 

 Eustachius (tuba auditiva [Eustachii]) . Crossing the cavity from the membrane 

 of the tympanum to its inner-wall is a chain of three auditory ossicles by 

 way of which vibrations of the tympanic membrane are transmitted to the 

 internal ear. The air-containing cavity of the osseous bulla of the temporal 

 bone is in free communication with the tympanic cavity. 



The internal ear consists of a series of spaces and canals within the tem- 

 poral bone, to which is given the collective name of osseous labyrinth (labyrinthus 



